1. Field of the Invention
The present invention relates to a poly-silicon (poly-Si) thin film transistor (TFT) having back bias effects and a fabrication method thereof, and more particularly, to a poly-Si TFT and a fabrication method thereof, in which a conducting underlayer or an island type buried electrode pad is formed on the lower portion of the TFT in order to apply a back bias, to thereby enhance characteristics of a leakage current, a sub-threshold slope, and an on-current, and revealing a stable operational performance.
2. Description of the Related Art
In general, a poly-crystalline thin film transistor (TFT) is used as a device for driving an active matrix liquid crystal display (LCD).
By the way, since a LCD uses glass as a substrate, it is required that a crystalline temperature for crystallizing an amorphous silicon film used as an active region into a poly-Si film be lowered.
Recently, a new method called a metal induced lateral crystallization (MILC) method has been developed as a method for crystallizing amorphous silicon deposited by a low-pressure vapor deposition method in order to fabricate a TFT at a low temperature of 500° C. or below.
When an amorphous silicon thin film is crystallized by a thermal treatment, the above method uses a silicide reaction between an amorphous silicon and a metal such as nickel (Ni), to thereby obtain an excellent poly-Si TFT of a large crystal particle under the temperature of 500° C. or below.
The above conventional poly-Si TFT fabricated by the Ni Offset and the MILC method has an excellent transistor characteristic.
However, in the result of measuring the transistor characteristic, the TFT reveals a leakage current value of ˜10−7 A/μm in the case that a gate voltage is 0V and when a drain voltage is high, that is, VDS>10V.
This value is a leakage current value larger by about one-thousand times as ˜10−10 A/μm which requires to drive an active matrix liquid crystal display (AM-LCD) using a poly-Si TFT.
Thus, it is essential to fabricate a transistor having a lower leakage current characteristic in order to apply a transistor fabricated by the MILC.
Also, since a voltage is not applied to a channel region of a device in the case of a transistor formed on an insulation substrate as in a poly-Si TFT, a device operational performance is degenerated.